Peter Salenieks - "I took these photos recently, during an ascent of Ojos del Salado in the Atacama Desert, Chile. The extent and luminance were remarkable – significantly greater than anything I’ve seen before in the mountains." Images ©Peter Salenieks, shown with permission
Luminous colour tendrils vein the sky.
Iridescent clouds? No, they are too bright. Too colour structured. Their colours change in spectral order from reds at top through orange and green to blues. The wider angle view at left tells us more. The colours are in a band far beneath a high sun.
A circumhorizon arc. An ice halo. Sunlight glints through ice crystals floating in the high cirrus cloud above the lower clumpy cumulus. The cirrus is in streaks that glow in spectral colours as their crystals glint. The lower patchy cumulus helps also to block the halo. These fragments of the complete halo render it enigmatic and exceptionally beautiful.
Circumhorizon arcs need optically near perfect ice crystals in the form of hexagonal plates. The plates naturally align horizontally as they drift slowly downwards in the cloud's air currents. They do not fall from the cloud as many accounts would have you believe - they are the cloud!
Sunlight enters a vertical side face and leaves the lower face. The double refraction through effectively a 90 degree prism disperses the sunlight colours widely - and purely.
The circumhorizon arc is closely related to a circumzenithal arc. To form the latter, light enters the top face and leaves through a side face.